1. Field of the Invention
The present invention relates to altering the coefficient of thermal expansion of materials, including gases, liquids and solids, and, more specifically, the introduction of filler materials into these materials to control the overall coefficient of thermal expansion.
2. Description of the Related Art
In many areas of technology, such as microelectronic packaging, there are circumstances where a mismatch of coefficient of thermal expansion (CTE) between two adjacent materials causes undesirable stresses, which can lead to interfacial failure. This problem has been addressed in many ways. For example, one material can be modified by a filler material to form a composite having a CTE that is more compatible with that of an adjacent material. A simple example might be to add an SiO2 filler to epoxy glue to reduce the epoxy glue's composite CTE to be closer to that of a metal to which it will be adhered.
The problems associated with CTE mismatch are often addressed by the incorporation of many different types of fillers. However, the extent to which the CTE of a material can be thus reduced is limited. The common filler SiO2 has a CTE of about 2 ppm, which is very low compared to organic materials which ordinarily have CTEs in the 50 to 200 ppm range. Only limited volume percent filler can be added before properties other than CTE become significantly affected, which is often undesirable.
A more exotic class of filler used to lower the composite CTE includes those with negative CTEs. Most of these are anisotropic and thus can only impart the desired CTE contribution in one direction. Graphite and Kevlar fibers are examples of negative CTE filler materials. Some very rare materials have volumetrically negative CTEs and the incorporation of these materials can impart lower composite CTEs relative to SiO2. However, most of the volumetrically negative CTE materials have CTEs just slightly below zero ppm, at about −1 ppm, and thus the improvement over SiO2 is only moderate in light of the high cost of these materials. An example of a material having a volumetrically negative CTE is Zirconium Tungstate (ZrW2O8), produced by Wah Chang of Albany, Oreg.
Thus, a need exists for a relatively inexpensive and highly negative CTE material that can be mixed as a composite with other materials to significantly lower the composite CTE.